The present invention is directed to a method of decompressing images compressed in accordance with the currently proposed JPEG ADCT (adaptive discrete cosine transform) standard, and more particularly, a method of reducing decompression ringing artifacts in document-type images resulting from decompression of standard JPEG ADCT compressed images.
Data compression is commonly used in data handling processes where too much data is present for practical storage or transmission of the data. For example, compression is used in communication links, where the time to transmit is relatively long, or where bandwidth is limited. Another use for compression is in data storage, where the amount of media space on which the data is stored can be substantially reduced with compression. Yet another application is a digital copier where an intermediate storage is useful for collation, reprint or any other digital copier functions. Generally speaking, scanned images, i.e., electronic representations of hard copy documents, are commonly large, and thus are desirable candidates for compression.
Many different compression techniques exist, and many are proprietary to individual users. However, standards are desirable whenever intercommunication between devices will be practiced. Particularly with the advent of multimedia communication, where formerly dissimilar devices are required to communicate, a common standard will be required. An example is the current desirability of FAX machines to be able to communicate with printers. Currently, compression standards are generally distinct for different devices.
ADCT (Adaptive Discrete Cosine Transform, described for example, by W. H. Chen and C. H. Smith, in “Adaptive Coding of Monochrome and Color images”, IEEE Trans. Comm., Vol. COM-25, pp. 1285-1292, November 1977), as the method disseminated by the JPEG committee will be called in this application, is a lossy system which reduces data redundancies based on pixel to pixel correlations. The ADCT process and some of its problems related to document images are discussed in U.S. Pat. No. 5,379,122 to Eschbach, incorporated herein by reference in its entirety. In particular, the process cannot reproduce the original image since data within the image was discarded in a compression-quantization step. Failures are noted wherever strong edges, commonly present in text, appear. Particularly, at such edges “ringing artifacts” or in some references, “mosquito noise” is noted. These problems occur in text and graphics, components very common in document images. In addition to mosquito noise or ringing artifacts, a blocking artifact often appears which is associated with image areas having slowly varying grays, where each M×M block into which the image was divided, and which formed the calculation of the compression basis, becomes visually distinguishable from adjacent blocks.
Eschbach describes a method of decompressing a document image which removes or lessens the effects of the artifacts noted above. Although Eschbach's method cannot accurately determine the original image because that image has been lost, the method does render the decompressed image more visually appealing, and creates an image which is also one of the images in a set of images which would have produced the same set of quantized ADCT values, and thus could be the original image. Eschbach's method is, however, computationally intensive.